This chromatograph is reproduced in Figure 74 [Image could not be located] The use of, or combination of, other diagnostic aids may also prove valuable. Thus, Albach and Redman used the composition of flavones in citrus fruits as an aid in citrus classification, but these compounds might be a factor in bark identification as well. Dreyer used citrus fruit bitter principles for chemotaxonomy in the Rutaceae. Esen and Soost found that based on the occurrence or absence of browning in young shoot extracts, Citrus taxa can be classified into two phenotypes: browning and nonbrowning. The ability to cause browning in shoot extracts has been shown to be due to a single gene , which controls the production of a substrate of polyphenol oxidase. Esen and Soost suggest the technique might not only be useful as a genetic marker, but also as a taxonomic criterion, when used with other procedures. Another very helpful chemical technique to aid in citrus identification, particularly in distinguishing between nucellar and zygotic seedlings, is the use of isozymes. This procedure is based on the horizontal starch gel electrophoresis of heterozygous loci found in leaf extracts of the cultivars to be analyzed,cultivo frambuesa whether known cultivars or new hybrid progenies. Perhaps the first researchers to use the method as a practical approach in Citrus were Ueno , Ueno and Nishiura , and Ueno and Nishiura . Ueno and Nishiura were very successful in identifying hybrid and nucellar seedlings in progeny obtained from a breeding program extending over a 10 year period at the Fruit Tree Research Station at Okitsu, Japan. The progeny of the crosses were categorized by analysis of the leaf enzymes using peroxidase isozyme electrophoresis.
Ueno extended this technique to confirm the identity of Citrus species and varieties in the collection at the Okitsu Station. Ueno and Nishiura used the same procedure to study the graft hybrids, Kobayashi mikan , Kinkoji-unshiu , and Takagi-mikan . They were able to establish that the Kobayashi mikan and the Kinkoji unshiu were tree graft hybrids, but that the Takagi-mikan was not. Torres, Soost, and Diedenhofen were the first to report clearly the allozyme systems in Citrus. They reported 19 codominant identifiable alleles distributed among four loci controlling three enzymes. These three gene enzyme systems were glutamate oxaloacetate transaminase , phosphoglucose isomerase , and phosphoglucose mutase . Using this technique, Soost, Williams, and Torres were clearly able to distinguish between zygotic and nucellar five-month-old citrus seedlings. The seedlings were from a cross using King mandarin as the female parent and Parson’s Special mandarin as the pollen parent. Of the 128 seedlings obtained from the cross, and using the two genetically defined markers , they found 18 to be nucellar and 110 to be zygotic. Fortunately, all of these seedlings were planted in the orchard for further observations. The work of Soost and Torres extends and amplifies the work of Torres, Soost and Diedenhofen . Three additional gene-enzyme systems, malate dehydrogenase , hexose kinase and isocitrate dehydrogenase , were determined and used for possible identification of cultivars in the subgenus Eucitrus. Additional taxa were analyzed for glutamate oxaloacetate transaminase , phosphoglucose isomerase and phosphoglucose mutase . Examples of the use of the three latter enzyme systems are presented in the paper as well as considerable references to the numerous cultivars in the Citrus Variety Collection at the Citrus Research Center, Riverside.
Torres, Soost, and Mau-Lastovicka emphasize that isozymes of Citrus provide molecular tags to determine the genetic origin of citrus seedlings. A very large proportion of all possible seedlings, either from selfing or crossing, can now be distinguished with a great deal of certainty as to their genetic origin. Citrus leaf extracts were analyzed by starch gel electrophoresis for the previously mentioned enzymes of hexokinase , isocitrate dehydrogenase , leucine aminopeptidase , malate dehydrogenase and malic enzyme . The isozyme technique is wonderful for distinguishing between nucellar and zygotic seedlings, but for other uses it does have limitations. Sometimes it may accurately identify the genetic makeup of many cultivars, in others, not. However, in some cases it is extremely helpful in that although it cannot tell one of the parentage of a cultivar, it can tell one what it is not. Sibs out of the same cross cannot be identified from each other. Orange cultivars cannot be accurately identified, or with little confidence. Carrizo and Troyer citrange have not been definitely identified. Eureka, Lisbon, and Villafranca lemon strains cannot be distinguished, etc. Many of the various chemical tests are rather complicated and not only need special supplies and equipment, but also trained technicians. If such tests are necessary, the grower should obtain the services of a professional laboratory with experience in these techniques. Furthermore, these tests are not practical for use by the nurserymen who grow thousands of trees or hundreds of thousands of trees. One should use root stocks which are highly nucellar, such as Troyer and Carrizo citrange, Rough lemon, sweet orange, etc. Using root stocks like Sacaton citrumelo and Citrus taiwanica, which only have about a 50% nucellar rate, can provide nothing but trouble. The nurseryman cannot possibly effectively discard all the zygotes and hence, tree variation and poor performance in the orchard is to be affected. The sweet orange, Citrus sinensis, achieved prominence only in California .
In Southern California during the period from 1900-1950 it was probably the principal root stock in use, except on heavy soils and the sandy soils of the Coachella Valley. Since 1950, its use as a root stock has steadily declined so that by about 1970 it was seldom planted, but is in certain situations. As a root stock, its performance in California was very satisfactory, with all major commercial scions performing comparable to those same scion trees on sour orange or better. Such trees were superior in performance to trees on Rough lemon, grapefruit, Cleopatra mandarin, trifoliate orange, etc., see Webber , Wutscher , and Castle . Sweet orange was used to a very limited extent in Florida and Australia. It was well tested in South Africa by Marloth , but never achieved acceptance. The sweet orange consists of a homogeneous group of cultivars, one of the most uniform of the citrus species. Maximum variability may exist between the Valencia and the navel, the red- pigmented “blood” oranges , the pink pigmented varieties , the blond oranges, the acidless oranges, and the seedy varieties that have been used for root stocks. They are commonly and collectively referred to in California as ‘sweets,’ ‘seedy sweets,’ or ‘Mediterranean sweets.’ According to origin, they may be ‘Blackman’, ‘Koethen’, ‘Hinckley’, ‘East Highland’, ‘Olivelands’, etc. In Florida, seedy varieties like ‘Pineapple,’ ‘Parson Brown,’ ‘Homosassa,’ and ‘Florida common’ were some of the sources used. The sweet orange varieties grow readily from seed,plastic gutter but the seeds are easily injured by drying out and must be handled carefully . All sweet orange varieties are highly nucellar , and require a minimum amount of rogueing in the seed beds and nursery seedlings to remove the variants. The seedlings grow more slowly, when young, than those of the sour orange, and produce low-branching bushy trunks which may require more shaping in the nursery prior to budding than seedlings of Rough lemon, Troyer citrange, Cleopatra mandarin and others. The scion buds, after insertion, grow rapidly and produce large budlings with most scions —but not as large as on Rough lemon, Rangpur lime, Alemow, etc. The sweet orange may be grown from cuttings more easily than the sour orange, but not as easily as cuttings of the Rough lemon. As a root stock, the sweet orange in California is only medium in cold resistance, more hardy than Rough lemon, but maybe not as good as sour orange. When mature trees are frozen to the ground, as in a Florida freeze, it sprouts readily from the base of the trunk.The sweet orange usually gives normal bud unions with all varieties of sweet orange, mandarin, grapefruit, lemon and lime. That is to say the stock and scion are usually nearly equal in size, particularly with young trees. With older trees, there may be a tendency for a slight bulge at the union and the stock may be slightly smaller than the scion, but not to the degree attained on sour orange stock. This is generally true with grapefruit and lemon scions. In some instances, Eureka lemon trees on sweet orange may show a slight bud union overgrowth. For illustrations of these bud unions, see Figure 76 [Image could not be located]. The sweet orange grows fairly well on some heavy soils, but is best adapted to growth on rich sandy loams. Trees on sweet orange do not do well on very sandy soils such as in the Coachella Valley of California, nor extremely heavy or calcareous soils such as Porterville or Ducor adobes. On heavier soils that are poorly drained, the trees may develop severe iron chlorosis symptoms, and this is also true on the calcareous soils. Embleton, et al. and Wutscher found that trees on sweet orange root may have higher leaf levels of nitrogen, phosphorous and copper than trees on other root stocks.
The trees of sweet orange root stock do not commonly develop a well differentiated tap root like the sour orange root stock and is usually moderately shallow rooted, rarely penetrating to the depth that sour orange root stocks do, although an occasional lateral may do so. The sweet orange does, however, develop an abundant system of lateral roots which generally penetrate deeper than those of Rough lemon in California soils. Illustrations of these root systems may be found in the section on roots. Navel orange cuttings produced only a few very large surface laterals which have little penetration. On the other hand, Valencia orange cuttings produce a more abundant root system similar to those obtained on sweet orange seedlings. There would appear to be no disadvantage in using Valencia orange cuttings for orchard trees in areas where sweet orange could satisfactorily be used as a root stock. Trees on sweet orange root stock are large and vigorous, producing standard sizes in combination with all commercial citrus varieties in California. In most areas of California, the sweet orange trees are larger than those on sour orange root stock, but are smaller than trees on Sampson tangelo or Troyer citrange. Also, in California, they have, on the better soils, produced larger trees than those on Rough lemon, but in the sandy soils of Florida they are generally smaller. Yields on sweet orange root stock are good, generally among the higher echelon with all scion varieties except with navel oranges, where trees on sour orange, Troyer citrange, or the non-commercial Morton citrange out-yielded them. Rios Castaño, Torres, and Camacho report that orange trees on sweet orange root stock in Columbia were low in production, but offer no explanation. The sweet orange combinations are not as precocious in bearing as trees on trifoliate orange or Alemow, but the trees under favorable situations are long-lived and bear well into the advanced age of 50-60 years or longer. One orchard at the Citrus Experiment Station, Riverside must be 80 years and is still productive, although tree size is getting out of hand. There have been few loses from gummosis in the old orchard, but some trees have declined from psorosis. In California, fruits on the sweet orange stocks mature at the normal season for the variety; they are thin skinned, juicy, and of high quality, and hold up well in all physical and chemical characters to the extreme end of the long harvesting season. Percent juice, soluble solids and citric acid content of the fruits are essentially identical to those obtained on sour orange—with all varieties and in all areas of California . Wutscher , however, reports that the acid content of fruit on sweet orange root stock in Texas was higher than that of fruit on sour orange. This is not true in California. Fruits on sweet orange root stock are thus intermediate in quality, being superior to those grown on Rough lemon, sweet lime, or Alemow. They are, however, of poorer quality than those grown on trifoliate orange or Troyer and Savage citranges. Granulation of the fruit is generally not a serious problem as compared to other stocks.